Two very distinct types of T lymphocytes, expressing either an alpha/beta (ab) or gamma/delta (gd) T cell receptor (TCR), are involved in immune responses. Knowledge of the antigens that gd T cells recognize and their specific functions lags extensively behind that of ab T cells. Although not well defined, gd T cells are known to play roles in immune surveillance, regulation of adaptive immune responses, and the specific targeting of infected cells. They also have potent anti-tumor responses and are involved in both promoting and preventing autoimmunity. These roles are poorly defined largely because the antigens recognized by specific gd TCRs are not known. Gd TCRs are typically considered self-reactive, responding to stress-induced host molecules, although some are known to recognize pathogen-derived molecules. Our laboratory has developed a novel technique to detect host-derived ligands on cells using purified, soluble forms of specific gd TCRs as staining reagents. Soluble TCRs (sTCRs) will provide a novel approach to the identification of specific cell surface ligands responsible for activating gd T cells. I propose to use sTCRs to identify the ligands for two common murine gd TCRs, Vg1:Vd6.3 and Vg6:Vd1, and to demonstrate the role of the TCR in activating the responses of gd T cells bearing these receptors. I will also confirm that the discovered ligands are the physiological targets of the corresponding gd cells. Vg1:Vd6.3 and Vg6:Vd1 gd cells play immunoregulatoy roles during infections. Therefore, the identification of these ligands will promote further research and a better understanding of the roles of gd cells in inflammation. As well, identification of additional gd TCR ligands will provide insight into the specificity requirements of the gd TCR. We also hope the method used to identify these ligands will be useful for researchers to identify ligands for many gd TCRs of interest and lead to a better understand of the roles of gd T cells in a wide variety of immune responses. Gd T cells are important players in the body's defense against certain bacteria, viruses, tumor cells, and itself. In order to understand how gd cells are involved in these defenses, we need to identify the molecules that stimulate them to respond. The work proposed here will lead to the identification of these molecules.